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There are a number of different guidelines and scoring systems available, which have different ways of identifying the ideal ‘30% total crop’ dry matter stage. Some of these can be very complex and although no doubt effective, are difficult to use without some degree of experience or training. Often the stages of development are hard to identify in practice and can also require some assessment of the size and greenness of the overall plant. In the hands of an expert, such systems are very effective and possibly more precise that the simple system provided below. The key below, however, is intended as a simple guide that is very easy to remember and use.

[ < means less than, > means greater than]
*30 - 35% DM Harvest stage

This simple system of scoring cob maturity comprises only five main categories, which are visibly easy to distinguish and will help growers gage how quickly a maize crop is progressing towards harvest.

Watery milk: This is when the cobs are white and the juice runs freely out of the kernels when punctured with a fingernail.

Creamy milk: This is when the cobs are showing some cream to pale lemon colouring but when punctured with the fingernail, a cloudy juice runs freely from the kernel to leave little or no solids.

Starch forming: This is the most important category as it is when the maize plant is laying down starch and spans the period from early filling to the early stage of maturity when the kernel contains cheesy starch but still releases a little moisture when punctured. This category has been divided into three simple stages depending on the ratio of milk to starch in the kernel.

The ideal harvest mature stage of 30-35% dry matter content will occur during the next stage and speed of change and timing of harvest depends on the following conditions:

This simple scheme should be very easy to use and has given very good agreement between different users. Particularly during the important ‘Starch Forming’ category, it should be a simple task to decide whether or not there is more starch than milk in the kernels. Even if the kernels do not exactly fit the description above, there are midway scores that can be used to show this is the case.

As temperatures fall in October, so development changes slow down, but crops need to be checked frequently once Stage 7 (milk < starch) is reached. Once the cobs are internally dry it is expected that harvest should follow, though where an excessively large and green standing crop has been built up some delay may be reasonable to allow the rest of the plant to dry out. Care must be taken not to over mature the cobs towards Stage 9 (Floury) due to the associated spoilage and utilization risks plus the problem that a heavily senesced crop will have depressed ME levels. So the key stage for harvest is when the kernel is a firm ‘juiceless cheese’. If too hard and mealy and turning a glazed orange, it is over ripe and if milky when squeezed, then hold off harvesting a little longer to gain more starch and a higher dry matter.

Harvesting before the correct stage is unlikely to deliver any benefits other than a completion of the harvesting operation and is likely to sacrifice additional starch that the maize could have produced if allowed to stand longer.

This Crossnacreevy cob key is intended only as an aid to correctly timing of maize harvests. It will provide a reliable indicator of when it is safe to leave the crop growing on to mature further or when the crop will need to be closely watched and a harvest decision taken soon.